Possible misaligned orbits in the young planetary system AU Mic

The AU Microscopii planetary system is only 22 Myr old, and its geometry may provide clues about the early dynamical history of planetary systems. Here, we present the first measurement of the Rossiter-McLaughlin effect for the warm sub-Neptune AU Mic c, using two transits observed simultaneously with VLT/ESPRESSO, CHEOPS, and NGTS. After correcting for flares and for the magnetic activity of the host star, and accounting for transit-timing variations, we find the sky-projected spin-orbit angle of planet c to be in the range of 67.8 -49.0/+31.7 degrees (1-sigma). We examine the possibility that planet c is misaligned with respect to the orbit of the other known planet in the system and the equatorial plane of the host star, and discuss scenarios that could explain both this and the planet’s high density, including secular interactions with other bodies in the system or a giant impact. Further observations will be crucial to constrain the inclination of AU Mic c’s orbit more precisely.

A Gaussian process model for stellar activity in 2-D line profile time-series

Stellar active regions like spots and faculae can distort the shapes of spectral lines, inducing variations in the radial velocities (RVs) that are often orders of magnitude larger than the signals from Earth-like planets. Efforts to mitigate these activity signals have hitherto focused on either the time or the velocity domains. We present a physics-driven Gaussian process (GP) framework to model activity signals directly in time-series of line profiles or Cross-Correlation Functions (CCFs). Unlike existing methods which correct activity signals in CCF time series, our approach exploits the time-correlation between velocity bins in the line profile variations, and is based on a highly simplified but physically motivated model for the origin of these variations. When tested on both synthetic and real data sets with signal-to-noise ratios down to ~100, our method successfully separated planetary from activity signals, even when their periods were identical. We also present injection/recovery tests using 2 years of realistically sampled HARPS-N solar data, which demonstrate our ability to accurately recover planetary signals down to 0.3 m/s and out to 33-day period during high solar activity.

Modelling stellar variability in archival HARPS data: I – Rotation and activity properties with multi-dimensional Gaussian Processes

Although instruments for measuring the radial velocities (RVs) of stars now routinely reach sub-meter per second accuracy, the detection of low-mass planets is still very challenging. The rotational modulation and evolution of spots and/or faculae can induce variations in the RVs at the level of a few m/s in Sun-like stars. To overcome this, a multi-dimensional Gaussian Process (GP) framework has been developed to model the stellar activity signal using spectroscopic activity indicators together with the RVs. A recently published computationally efficient implementation of this framework, S+LEAF2, enables the rapid analysis of large samples of targets with sizeable data sets. In this work, we apply this framework to HARPS observations of 268 well-observed targets with precisely determined stellar parameters. Our long-term goal is to quantify the effectiveness of this framework to model and mitigate activity signals for stars of different spectral types and activity levels. In this first project in the series, we initially focus on the activity indicators (S-index and Bisector Inverse Slope), and use them to a) measure rotation periods for 49 slow rotators in our sample, b) explore the impact of these results on the spin-down of middle-aged late F, G & K stars, and c) explore indirectly how the spot to facular ratio varies across our sample. Our results should provide valuable clues for planning future RV planet surveys such as the Terra Hunting Experiment (THE) or the PLATO ground-based follow-up observations program, and help fine-tune current stellar structure and evolution models. For more details, please see Yu et al. (2024).

Mapping the 3D Kinematical Structure of the Gas Disk of HD 169142 (finding infant planets in protoplanetary disks!)

The disk around HD 169142 has been suggested to host multiple embedded planets due to the range of structures observed in the dust distributions. We analyze archival Atacama Large (sub-) Millimetre Array observations of  ¹²CO (2–1), ¹³CO (2–1), and C¹⁸O (2–1) to search for large-scale kinematic structures associated with other embedded planets in the outer disk. At 125 au, we identify a coherent flow from the disk surface to the midplane, traced by all three CO isotopologues, and interpret it as a meridional flow potentially driven by an embedded planet. We use changes in the rotation speed of the gas to characterize the physical structure across this region, finding that at 125 au the CO emission traces regions of increased gas pressure, despite being at a surface density minimum. Developing a simple analytical model, we demonstrate that the physical structure of the gap can have non-trivial responses to changes in the surface density, consistent both with previous thermo-chemical models and the conditions inferred observationally. Applying this technique to a range of sources will allow us to directly confront theoretical models of gap-opening in protoplanetary disks. For more details, please see Yu et al. (2021).